It is obviously necessary for lubrication to exist in the form of an oil film between the valve stem and guide, and in some engines a large clearance exists between valve stem and guide such that the lubricant can pass the valve stem and enter the combustion chamber. Standard tolerances for the average "free running fit" diameters, such as might otherwise be commonly encountered in air cooled motor cycle engines, range from a maximum of 0.003 inches (United States standards) to a maximum of 0.0018 inches (British standards), and yet is known that some motorcycles have the minimum valve stem clearance of 0.003 inches, and quite often the minimum recommended clearance is 0.005 inches. Such clearances are sufficient to allow oil to enter the combustion chamber, but this is undesirable for the following reasons:
a. smoke emissions from the exhaust; PA1 b. fouling of spark plugs; PA1 c. excess carbon build up on piston crowns which may glow red hot causing pre-ignition; PA1 d. carbon particles depositing between the valve head seat and valve and cause valves to burn out; PA1 e. depletion of sump oil; PA1 f. interference with correct air/fuel mixture which can cause loss of power; and PA1 g. contamination of sump oil which can result in overall reduced engine life.
This is a problem which has attracted a great deal of attention by engine designers, and there is a large amount of recorded art which relates to the design and lubrication of valve stems and guides. Notwithstanding all the efforts which have been made, however, it is still not uncommon for engines to encounter valve difficulties, and bent or otherwise damaged valve stems are frequently the cause of engine failure.
If lubricant does not enter the valve guide around the valve stem, the likelihood of engine failure due to seizure of the valve stem within its guide is very high.
Thus a problem exists in ensuring that some lubricant is always present throughout the whole of the length of a valve stem and guide, and yet to ensure that the amount of lubricant which finally reaches the combustion chamber is minimal.